Not only is a synthetic silica glass transparent over a broad range of wavelengths from infrared light to vacuum ultraviolet rays, it is excellent in thermal stability and chemical stability, and is also widely used for optical fibers for communication and for fibers for laser oscillation (in the following also referred to as “fiber laser”). Compared to a bulk laser, the fiber laser has many advantageous aspects such as stability of the laser oscillation and being able to reduce the size of equipment, and increasing attention is being paid to it.
The fiber laser contains dopants such as Al or rare earth elements, for example Er or Yb, in a core section to a percentage level, and provides a laser oscillation function (as disclosed for example in Japanese Unexamined Patent Application Publication 2009-78928)). Excited light that is transmitted through the core section is amplified and oscillated at a fiber end. A cladding is formed around the core so that the excited light does not diffuse outside the fiber. At that time, a refractive index of the cladding is set lower than the refractive index of the core such that light facing outward from the core is not inclined and diffused outward.
With regard to a laser oscillation output, year by year, there is a tendency that a high output is required, and the concentration of metallic elements that are doped in order to intensify the intensity of excited light has also become higher. When the doping metal concentration of the core section increases, the refractive index increases. Therefore, in order to keep the inclination angle at an appropriate level, it is also necessary to increase the refractive index of the cladding.
The refractive index is dependent on electron density; therefore, the greater the electron density, the greater the refractive index. When there is electrical non-uniformity in a silica glass, the refractive index increases correspondingly. For example, the metallic elements of Group 3 or 13 that are doped in the silica glass easily carry electrical charges, and are known as refractive index enhancement element. However, simply by doping the metallic elements, the doping concentration was required to be maintained up to the percentage level, there were numerous problems in the manufacturing method and resulting glass quality, and particularly, there was a huge problem with high doping to the synthetic silica glass. Specifically, when the metallic elements were doped to the percentage level in order to increase the refractive index of the cladding to a desired level, for a doped synthetic silica glass for cladding, foams and bubbles are generated and other foreign substances occur, and after drawing of the fiber, the breaking strength deteriorated significantly, such as causing multiple fractures, and furthermore, optical degradation such as photodarkening occurred; therefore, there were many technical problems.